Development of high-brightness LEDs, and their incorporation into Lamps designed to replace incandescent bulbs has revolutionized the lighting industry in recent years. One of the advantages of an LED lamp over an incandescent lamp is its greater efficiency in converting electric energy into light. A typical incandescent bulb produces about 14-17.5 lumens per watt, and most halogen lamps produce about 16-21 lumens per watt. In comparison, LEDs achieving 80-100 lumens per watt are now common. Even when considering the power that is lost in the driving circuitry of an LED lamp which may be 60-80% efficient, LED lamps that are three to six times as efficient as incandescent and halogen bulbs are easily achievable. Thus an LED lamp designed to replace a halogen bulb in a line-voltage fixture would draw much less power from the AC mains than the halogen bulb for which the light fixture was designed. In installations employing many such light fixtures, a great savings in electric energy can be realized by replacing the halogen bulbs with a comparable LED Lamp.
LEDs are current-controlled devices in the sense that the intensity of the light emitted from an LED is related to the amount of current driven through the LED. FIG. 1 shows a typical relationship of relative luminosity to forward current in an LED. The longevity or useful life of LEDs is specified in terms of acceptable long-term light output degradation. Light output degradation of LEDs is primarily a function of current density over the elapsed on-time period. LEDs driven at higher levels of forward current will degrade faster, and therefore have a shorter useful life, than the same LEDs driven at lower levels of forward current. It therefore is advantageous in LED lighting systems to carefully and reliably control the amount of current through the LEDs in order to achieve the desired illumination intensity while also maximizing the life of the LEDs.
LED driving circuits, and any circuit which is designed to regulate the power delivered to a load can generally be categorized as either linear or active. Both types of circuits limit either the voltage, or current (or both) delivered to the load, and regulate it over a range of changing input conditions. For example, in an automotive environment the voltage available to an LED driving circuit can range from 9V to 15 Vdc. A regulator circuit is employed to keep the current delivered to the LEDs at a relatively constant rate over this wide input range so that the LED output intensity does not noticeably vary with every fluctuation in the system voltage.
Linear regulators are one type of device or circuit commonly employed to accomplish this task. A linear regulator keeps its output in regulation only as long as the input voltage is greater than the required output voltage plus a required overhead (dropout voltage). Once the input to the regulator drops below this voltage, the regulator drops out of regulation and its output lowers in response to a lowering input. In a linear regulation circuit, the input current drawn by the circuit is the same as the output current supplied to the load (plus a negligible amount of current consumed in the regulator itself). As the input voltage presented to the linear regulator rises, the excess power delivered to the system is dissipated as heat in the regulator. When the input voltage is above the dropout threshold, the power dissipated in the regulator is directly proportional to the input voltage. For this reason, linear regulators are not very efficient circuits when the input voltage is much larger than the required output voltage. However, when this input to output difference is not too great, linear regulators can be sufficient, and are commonly used due to their simplicity, small size and low cost. Because linear regulators drop out of regulation when the input is below a certain operating threshold, they can also be employed in LED driving circuits to effect a crude dimming function in response to an input voltage which is intentionally lowered with the desire to reduce the LED intensity. The dimming is “crude” in that it is not a linear response for two reasons. First, in the upper ranges of the input voltage above the dropout threshold, the regulator will hold the output in regulation and the LEDs will not dim at all. Once the dropout threshold is reached, the output voltage will drop fairly linearly with a further drop in input. However, LEDs are not linear devices and small changes in voltage result in large changes in current which correspondingly effect large changes in output intensity. As the voltage applied to an LED is lowered below a certain threshold, no current will flow through the LED and no light will be produced. FIG. 2 is an example of a linear regulator circuit configured to drive an LED load. FIGS. 3 and 4 give an example of the response of this linear regulated LED circuit to a dimmed input voltage.
The lower power efficiency of linear regulators makes them a poor choice in large power systems and in systems where the input voltage is much larger than the required LED driving voltage, such as when a 120 Vac or 240 Vac line voltage is used to drive a small number of LEDs. As such, these systems can not practically employ them. As LEDs have increased in power and luminous output, it has become common to employ driving circuits that are active, meaning the power delivered to the end system is dynamically adapted to the requirements of the load, and over changing input conditions. This results in increased system efficiency and less heat dissipated by the driving circuitry. Such active driving circuits are commonly implemented using switching regulators configured as buck, boost, or buck-boost regulators with outputs that are set to constant-voltage, or constant-current depending on the circuit. Typically, in LED driving applications, the switching regulator circuit is adapted to sense the current through the LEDs, and dynamically adjust the output so as to achieve and maintain a constant current through the LEDs. FIG. 6 depicts a typical buck regulator circuit configured to drive an LED load at a constant current.
Many switching regulator devices have been specifically designed for driving high powered LEDs. Manufacturers have built into these devices, inputs which can be pulsed with a PWM (pulse width modulation) or PFM (pulse frequency modulation) control signal or other digital pulsing methods in order to effect a lowering of the output of the switching regulator specifically designed to dim the LEDs. Some devices also have analog inputs which lower the output to the LEDs in response to an input which is lowered over an analog range. With such dimming capabilities built into the switching regulators, very accurate linear dimming of the LEDs can be achieved. Such dimming can be controlled via a network, or some user interface which generates input signals that are converted to the required digital pulses or analog signals that are sent to the switching regulator driver. This method of dimming in LED lighting systems is common. However, it requires control circuitry and user interface equipment which adds a level of cost and complexity to the lighting system.
In many cases, lighting systems and wiring are already installed, and it is desired to replace these lights with LED lights. Or, it is desired to add LED lights to an existing system and have them work in harmony with lights and equipment which are not LED based. There are common household wall dimmers which are employed to dim incandescent lights, and there are high-end theatrical dimming systems which are used to dim entire lighting installations. These types of dimmers only affect the input voltage delivered to the Lights. There is no additional control signal which is sent to them. Therefore, LED lights which are designed to work in these systems must dim in response to a change in the input voltage.
As noted above, linear regulator based LED drivers will dim in response to a lowering of the input voltage. However the dimming is very non-linear and these regulators are not practical for use in line-voltage applications driving a small LED Lamp. Switching regulator drivers will also fall out of regulation and dim their output when the input voltage drops below a certain threshold, but as with linear regulators, when the input is above a threshold, their outputs will be held in regulation and the LED intensity will remain unchanged. This is an especially impractical method of dimming when there is a large difference between the nominal (undimmed) input voltage and the regulating threshold such as the line-voltage LED Lamp situation.
Another problem with dimming switching regulator drivers by lowering their input voltage below the regulating threshold is that these circuits need a certain start-up voltage to operate. Below this minimum voltage, the switching regulator either shuts off completely, or provides sporadic pulses to the LEDs as it attempts to start-up, or passes some leakage current to the LEDs which causes them to glow slightly and never dim to zero. In LED circuits employing multiple lights, each driver circuit can have slightly different thresholds, resulting in differing responses at low dimming ranges. As a result, some lights may flicker, some may be off and some may glow below the threshold voltage. This is unacceptable in most lighting systems that are required to dim using standard ac dimming controllers.
The Modified Dimming LED Driver patent application referenced above detailed an LED driver based on efficient switching regulators which provides smooth and linear dimming from 100% to off, in response to the dimming input voltage that is provided with industry standard ac dimmers.
The Adaptive Dimmable LED Lamp patent application, also referenced above, identified and resolved several unique difficulties arising when an LED Lamp is driven from an electronic transformer such as commonly found in track lighting and other low-voltage lighting fixtures. In these lighting fixtures, the low-voltage transformer interfaces with 120 Vac, 230 Vac, or 240 Vac line voltages, providing a lower (typically 12 Vac) voltage to the Lamp.
There are also installed lighting fixtures for small incandescent or halogen bulbs that do not employ a low-voltage transformer, but instead present the line-voltage directly to the bulb. An LED replacement bulb designed to retrofit into these fixtures requires an off-line power driver capable of regulated DC output current, low DC output voltage and near unity input power factor.
A flyback converter is one common way to achieve the high step-down conversion ratio required for operating low-voltage LEDs from a high input voltage. When operating in discontinuous conduction mode, a flyback converter inherently provides a good power factor since the peak current in its inductor is proportional to the instantaneous input voltage. However, a very large electrolytic smoothing capacitor is needed at the load in order to attenuate the rectified AC line ripple component of the output current. The low dynamic resistance of LEDs aggravates this problem even further. AC line ripple is undesirable in illumination applications due to some people's sensitivity to this frequency of flicker.
There are two problems with electrolytic capacitors in driver circuits for LED replacement bulbs. First, electrolytic capacitors have relatively short life cycles compared to the LEDs and other components in the circuit, and this life cycle is greatly affected by the ambient temperature surrounding the capacitor. Unlike incandescent bulbs which radiate much of the heat generated, an LED lamp must remove excess heat through conduction to the shell and then convection to the air (along with conduction to the fixture). Thus, there are high temperatures in the base of an LED Lamp, which is detrimental to the life of any electrolytic capacitors used in the driver circuitry.
The second problem with electrolytic capacitors is their physical size. These large components quickly consume the small space available in a bulb base, and in many cases (such as in small MR16 bulb bases) this prohibits their use altogether.
There are power conversion topologies that can resolve this problem by cascading converter stages using a single active switch. Most of these topologies include an input boost converter stage for shaping the input current. Hence they require a power transformer with a high step-down turn ratio in order to drive low voltage LEDs, even when galvanic isolation of the output in not required. Such a power transformer is also a large bulky device which is prohibited in the small space available in the base of an LED Lamp.
Because of the reasons discussed above, there is need in the industry for an LED lamp employing driving circuitry that can step down the high voltages of AC mains (90-260 Vac), where the driving circuit can be sufficiently miniaturized to fit into the base of a standard size bulb. There is also need for such an LED lamp to dim from full output to off when connected to typical AC dimmers, in a manner similar to halogen bulbs, such that the LED Lamp can be retrofitted into previously installed lighting fixtures. It is an object of the present invention to provide a complete LED lamp with integral dimmable driving circuitry such as that disclosed in the Modified Dimming LED Driver application referenced above, and which may be powered directly from standard line-voltage of 90 Vac-260 Vac. It is a further object of the present invention to incorporate such LED driving circuitry without the use of electrolytic capacitors or power transformers, so as to fit within the available size of standard bulb bases. It is a further object of the present invention to provide the Lamp in an industry standard MR16 size with a bi-pin GU10 base so as to be a replacement for halogen bulbs common in the lighting industry.